In a wireless network, a wireless device may communicate with one or more radio access nodes to send and/or receive information, such as voice traffic, data traffic, control signals, and so on. In some cases, the wireless device may have a connection with multiple radio access nodes of different link quality. A problem may arise where important control information is to be transmitted to the wireless device, but the link quality with a particular radio access node is weak. For example, in a Wideband Code Division Multiple Access (WCDMA) system, a wireless device in soft handover (SHO) is essentially power-controlled by the best uplink (UL) cell. If the best UL is a non-serving cell, it may be difficult to ensure that important control information is reliably received at the serving cell.
For example, channels such as the High Speed Dedicated Physical Control Channel (HS-DPCCH) and Enhanced Dedicated Physical Control Channel (E-DPCCH) may carry important control information, such as hybrid automatic repeat request (HARQ) and positive acknowledgement (ACK)/negative acknowledgement (NACK) feedback for high speed downlink packet access (HSDPA). In existing implementations, HS-DPCCH and E-DPCCH may have a legacy dedicated physical control channel (DPCCH) as their reference channel. Other channels, such as E-DPDCH, which carries general data payload as well as in-band scheduling, may also be set relative DPCCH. In the scenario described above, DPCCH may be power controlled by all the cells in an active set, including the non-serving cell. Difficulties may arise from using a channel that is essentially power controlled by a non-serving cell as a reference for channels carrying important control information that needs to be received in the serving cell. For example, it is difficult to ensure reliable reception of these channels at the serving cell, and thus receipt of the control information by the serving cell cannot be guaranteed. The problem of weak communication links becomes particularly pronounced when the imbalance between the best UL and downlink (DL) becomes large, such as for heterogeneous networks or multi-flow operation.
Deployment of low-power nodes (LPNs) is seen as a powerful tool to meet the ever-increasing demand for mobile broadband services. A LPN may correspond, for example, to a remote radio unit (RRU), pico, or micro base station, allowing expansion of network capacity in a cost-efficient way. A network consisting of traditional macro base stations and LPNs is referred to as a heterogeneous network. Two examples of use-cases for heterogeneous network deployment that may be envisioned are coverage holes and capacity enhancement for localized traffic hotspots.
FIG. 1 is a block diagram illustrating an embodiment of a network 100. Network 100 includes one or more wireless devices 110, radio network nodes 115, radio network controller 120, and core network nodes 130. Network 100 may be any suitable type of network. For example, network 100 may be a heterogeneous network of the kind described above, and network nodes 115 may be a mixture of macro nodes and LPNs. Wireless device 110 may communicate with a radio network node 115 over a wireless interface. For example, wireless device 110 may transmit wireless signals to radio network node 115, and/or receive wireless signals from radio network node 115. The wireless signals may contain voice traffic, data traffic, control signals, and/or any other suitable information.
Radio network node 115 may interface with radio network controller 120. Radio network controller 120 may control radio network node 115 and may provide certain radio resource management functions, mobility management functions, and/or other suitable functions. Radio network controller 120 may interface with core network node 130. In certain embodiments, radio network controller 120 may interface with core network node 130 via an interconnecting network. The interconnecting network may refer to any interconnecting system capable of transmitting audio, video, signals, data, messages, or any combination of the preceding. The interconnecting network may include all or a portion of a public switched telephone network (PSTN), a public or private data network, a local area network (LAN), a metropolitan area network (MAN), a wide area network (WAN), a local, regional, or global communication or computer network such as the Internet, a wireline or wireless network, an enterprise intranet, or any other suitable communication link, including combinations thereof.
Core network node 130 may manage the establishment of communication sessions and various other functionality for wireless device 110. Wireless device 110 may exchange certain signals with core network node 130 using the non-access stratum layer. In non-access stratum signaling, signals between wireless device 110 and core network node 130 may be transparently passed through the radio access network. Example embodiments of wireless device 110, radio network node 115, and a network node (such as radio network controller 120 or core network node 130) are described with respect to FIGS. 10, 11, and 12, respectively.
Wireless device 110 may communicate with multiple radio network nodes 115. The communication links between wireless device 110 and radio network nodes 115 may be of differing quality. If the link quality with a particular radio access node is weak, difficulties may arise in ensuring receipt of important control information. These problems are described in more detail below.
SHO, also referred to as macro diversity, and fast closed-loop power control are essential features of WCDMA and high speed packet access (HSPA). FIG. 2 illustrates a traditional HSPA deployment scenario with two radio access nodes 115A and 115B having similar transmit power levels. For example, network nodes 115A and 115B may both be macro nodes with similar transmit power levels. Ideally, a wireless device 110A moving from serving cell 115A to non-serving cell 115B would enter the SHO region 202 at point A 204. At point B 206, a serving cell change would occur. During a serving cell change, the non-serving cell becomes the serving cell and vice versa. For example, during a serving cell change macro node 115A, the current serving cell, would become the non-serving cell, and the current non-serving cell 115B would become the serving cell. At point C 208, wireless device 110A would leave the SHO region.
A radio network controller, such as radio network controller 120 described above in relation to FIG. 1, is in control of reconfigurations. This may imply rather long delays for performing a cell change. During SHO, wireless device 110A may be power-controlled by the best uplink cell. In the scenario illustrated in FIG. 2, network nodes 115A and 115B have roughly the same transmit power, so the optimal DL and UL cell borders will coincide, i.e., the path loss from wireless device 110A to network nodes 115A and 115B will be equal at point B 206. Hence, in an ideal setting, and from a static (long-term fading such as shadowing) point of view, the serving cell 115A would always have the best uplink. In practice, however, due to imperfections (e.g., reconfiguration delays) and fast fading, wireless device 110A might be power controlled by non-serving cell 115B during SHO. In such a case, problems may arise due to the weaker link between serving cell 115A and wireless device 110A. For example, receiving essential control channel information, such as hybrid automatic repeat request (HARQ), Positive acknowledgement (ACK)/negative acknowledgement (NACK) feedback for HSDPA, and scheduling information for enhanced uplink (EUL), may be problematic.
FIG. 3 illustrates a HSPA deployment scenario with two radio access nodes 115A, 115B having different transmit power levels. In the scenario illustrated in FIG. 3, radio access node 115A is a macro node, and radio access node 115B is a LPN. Since macro node 115A and LPN 115B have different transmit power levels, the UL and downlink (DL) cell borders may not necessarily coincide. For example, wireless device 110A has a smaller path loss to LPN 115B, while the strongest received power is from macro node 115A. In such a scenario, the UL is better served by LPN 115B while the DL is provided by serving macro node 115A.
In FIG. 3, the region between the equal path loss border and equal downlink received power (e.g., common pilot channel (CPICH) receive power) border may be referred to as an imbalance region. In this region, some fundamental problems may be encountered. For example, wireless device 110A in position A 302 would have macro node 115A as the serving cell, but be power controlled towards LPN 115B. Due to the UL-DL imbalance, the UL towards serving macro node 115A may be very weak. In such circumstances, important control information, such as EUL scheduling information or HS-DPCCH might not be reliably decoded in the serving cell.
This problem may be addressed to some extent by utilizing available RNC based cell selection offset parameters. By tuning the Cell Individual Offset (CIO) parameter, the handover border can be shifted towards the optimal UL border. Similarly, the IN_RANGE and OUT_RANGE parameters may be adjusted in order to extend the SHO region.
FIG. 4 illustrates SHO operation for HSPA in a heterogeneous deployment with range extension, in accordance with certain embodiments. Like FIG. 3, FIG. 4 includes two radio access nodes 115A and 115B having different transmit power levels. More particularly, radio access node 115A is a macro node, and radio access node 115B is a LPN. FIG. 4 illustrates the effect of adjustments to the CIO parameter described above. While the adjustments to the CIO parameter may be beneficial from a system performance point of view, in certain heterogeneous networks the power difference between macro node 115A and LPN 115B may be more than 10 dB. In practice, it is unlikely the CIO parameter will be set to more than 6 dB due to considerations such as DL signaling cost in terms of radio resource consumption. As a result, the imbalance region may not be eliminated by means of CIO setting.
During RAN #56 in September 2012, a study item (SI) was initiated on UMTS Heterogenous Networks. During the SI, many solutions had been proposed to address the problem of scheduling information and HS-DPCCH reception in the serving cell for UEs 110 in the imbalance region having macro node 115A as the serving cell (region B in FIG. 4). One proposed solution is to provide a new secondary pilot channel (DPCCH2), in the UL.